1. Field of the Invention
This invention is directed to a magnesium electrochemical cell containing as an active cathode material an α-MnO2 of specific chemical and physical properties which provides high cell capacity and increased cycle lifetime. The invention is further directed to a magnesium battery containing a cathode having the α-MnO2 as active ingredient.
2. Discussion of the Background
Lithium ion batteries have been in commercial use since 1991 and have been conventionally used as power sources for portable electronic devices. The technology associated with the construction and composition of the lithium ion battery (LIB) has been the subject of investigation and improvement and has matured to an extent where a state of art LIB battery is reported to have up to 700 Wh/L of energy density. However, even the most advanced LIB technology is not considered to be viable as a power source capable to meet the demands for a commercial electric vehicle (EV) in the future. For example, for a 300 mile range EV to have a power train equivalent to current conventional internal combustion engine vehicles, an EV battery pack having an energy density of approximately 2000 Wh/L is required. As this energy density is close to the theoretical limit of a lithium ion active material, technologies which can offer battery systems of higher energy density are under investigation.
Magnesium as a multivalent ion is an attractive alternate electrode material to lithium, which can potentially provide very high volumetric energy density. It has a highly negative standard potential of −2.375V vs. RHE, a low equivalent weight of 12.15 g/mole of electrons and a high melting point of 649° C. Compared to lithium, it is easy to handle, machine and dispose. Because of its greater relative abundance, it is lower in cost as a raw material than lithium and magnesium compounds are generally of lower toxicity than lithium compounds. All of these properties coupled with magnesium's reduced sensitivity to air and moisture compared to lithium, combine to make magnesium an attractive alternative to lithium as an anode material.
Magnesium (Mg) batteries are being researched as a candidate for post lithium-ion systems. They are expected to be high energy battery systems, due to the high volumetric capacity made available via the two electron transfer per Mg. However, a cathode active material compatible with magnesium and providing high capacity and durability is a subject of much ongoing investigation.
Examples of cathode active materials for magnesium electrochemical cells which are conventionally known include sulfur, MnO2 and a Chevrel compound having a formula MgxMo6Tn, wherein x is a number from 0 to 4, T is sulfur, selenium or tellurium, and n is 8.
The inventors have previously identified a K ion stabilized α-MnO2 as showing very high reversible capacity (US 2013/0004830 A1). However, the physical and chemical factors of α-MnO2 which affect the capacity of this cathode material and how to optimize those physical factors to improve cathodic performance has not been described.
Padhi et al. (U.S. 2011/0070487) describes a mixed manganese oxide used as a cathode material in electrochemical cells. The mixed manganese oxide in Padhi contains Mn2O3 and a manganese oxide in octahedral molecular sieve structure. One preferred manganese oxide of the octahedral molecular sieve structure in Padhi is cryptomelane, i.e., a K-stabilized α-MnO2. However, Padhi emphasizes the necessity to include Mn2O3 in the cathode material.
Xu et al. (U.S. 2005/0135993) describes an amorphous nanostructured cation-doped manganese oxide material useful as ion intercalation host materials for electrodes of rechargeable batteries. The cation considered in Xu is lithium, sodium, copper or any mixture of these cations. The manganese oxide compound has the formula of RxMnO2+y/2 (where R is a doped cation, and x and y are a value selected from 0 to 2). The manganese oxide reported by Xu is an amorphous material.
Feddrix et al. (U.S. Pat. No. 7,501,208) describes a doped manganese dioxide electrode material made electrolytically (EMD) or by a wet chemical method (CMD). The manganese dioxide described is preferably a γ-MnO2.
Rossouw et al. (U.S. Pat. No. 5,166,012) describes a hydrogen manganese oxide compound wherein the framework is a α-MnO2 structure which is hydrated such that the compound is of formula: MnO2.xH2O wherein X is from 0.005 to 0.3. A ratio of hydrogen cations to manganese cations is 1/10 to 2/10. The compound is made by acid leaching (including sulfuric acid) Li2O from a lithium manganese oxide compound at 70-100° C. with an acid content of 0.5 to 10 molar. Utility of the hydrogen manganese oxide compound as an active cathode material in an electrochemical is also described.
Lecerf et al. (U.S. Pat. No. 4,975,346) describes an electrochemical battery having a lithium anode and a cathode containing an α-manganese dioxide (crytomelane) and further containing lithium ions. The atomic ratio of Li/Mn is from 0.1 to 0.5. The material is produced by heating a mixture of α-manganese dioxide and a lithium compound to 300 to 400° C.
Atwater et al (U.S. Pat. No. 6,982,048) describes a potassium doped mixed metal oxide obtained by alloying MnO2 with potassium and lithium which is represented by the formula LixKyMn2O4. In one embodiment, a material of formula Li0.8K0.1Mn2O4 is described. The alloy is prepared by first preparing a doped manganese dioxide and then mixing this intermediate with a lithium compound and heat treating. Atwater is silent with regard to the crystal form of the manganese dioxide.
Takashi et al. (JP 2009070733) describes a modified Mn2O3 which is doped with Pd, La, Er, Rh or Pt. Utility of this material as a cathode active material for a fuel cell is described.
Toru (JP 2007018929) describes a Mn8O16 material containing crystalline water as a cathode component of a nonaqueous electrolyte lithium electrochemical cell. The crystal form of the manganese oxide is not described.
Yamamoto et al. (U.S. 2010/0196762) describes a manganese oxide obtained by reduction of potassium permanganate in hydrochloric acid, filtration and washing of the formed precipitate, then drying and heat treating at 300 to 400° C. This material is described as poorly crystalline and likely an amorphous manganese oxide, which is nonstoiciometric. Mg electrochemical cells are recited in Claims 14 to 16.
None of these references discloses or suggests a relationship of ionic radius of an ion to performance as a stabilizing ion for α-MnO2 or a molar ratio of the stabilizing ion to manganese as factors which affect cathodic performance.
The inventors are directing effort and resources to the study of cathode materials useful to produce a magnesium battery of sufficient capacity and cycle lifetime to be useful as a power source for utilities requiring a high capacity and high cycle lifetime. Particularly, the inventors are investigating the chemical and physical properties of α-MnO2 and the relationship of those properties to performance as an active cathode material in a magnesium cell or battery, preferably a rechargeable magnesium battery.
Therefore, an object of the present invention is to provide a magnesium cell containing an active cathode material which is suitable for utility as a battery having high capacity and high cycle lifetime.
A second object of the invention is to provide a rechargeable magnesium battery having high capacity and high cycle lifetime.